1. Field of the Invention
The present invention relates to a driving circuit for laser diode. More particularly, it relates to a driving circuit for laser diode which supplies controllable voltage to the laser diode to provide a variable output intensity of laser diode.
2. Description of the Prior Art
A driving circuit for laser diode which regulates the intensity of the laser output has been employed, which is called hereinafter an APC circuit. The APC circuit is composed from the devices of; sampling a fraction of light emitted from the laser diode, controlling a driving current to be inputted to a diode driver for laser diode by output signal from aforesaid sampling device in order to regulate the intensity of the laser output. According to the present invention, a controllable input voltage is employed instead of a reference voltage in the prior art to provide variable output intensity of the laser diode.
FIG. 4 illustrates a sample of a block diagram of an APC circuit in the prior art. It is composed from a photo detector (1), a current - voltage converter (2), a differential amplifier (3), a reference voltage generator (4), a diode driver (5) and a laser diode (6).
FIG. 5 demonstrate the characteristics of the driving current versus output intensity of a laser diode. It distinctly shows the fact that even when the same driving current is supplied to the laser diode (6), the obtained output intensity of laser diode is larger for lower temperature. This is the reason why the APC circuit is required, which operate to regulate an input current to be applied to laser diode (6) in order to maintain the output intensity of laser diode (6) at a constant level.
FIG. 4 is a sample of the APC circuit which has the function to maintain the constant output intensity of laser diode (6).
The photo detector (1) samples a fraction of light emitted from the laser diode (6). An output current of aforesaid photo detector (1) is converted to voltage by the current - voltage converter (2), then it is introduced to a differential amplifier (3).
For example, when output intensity of laser diode (6) is 1 mW, the photo detector generates a output current of about 0.1 mA through said sampling.
The output of the reference voltage generator (4) is supplied to the other terminal of the differential amplifier (3). Then the differential amplifier (3) boosts the difference between output of the reference voltage generator (4) and output voltage of the said current - voltage converter (2) in order to supply an output signal to a diode driver (5).
For example, the differential amplifier (3), with the reference voltage generator (4) of 0.8 V, may meet the below condition that: the output of differential amplifier (3) is -8 V when an input voltage of 0.8 V is applied from the current - voltage converter (3), it becomes -9 V when the input voltage is changed to 0.9 V, and
the output of said differential amplifier (3) varies to -7 V when it receives the input of 0.7 V from current - voltage converter (2). Thus, the output of differential amplifier increases or decreases according as the corresponding output of current - voltage converter (2) decreases or increases.
The diode driver (5) feeds the driving current to the laser diode (6).
FIG. 4 illustrates a feedback loop comprising laser diode (6), photo detector (1), current - voltage converter (2), differential amplifier (3), diode driver (5) and laser diode (6) to regulate the output intensity of laser diode (6).
The output intensity of laser diode (6) becomes stable according as the output voltage of the current - voltage converter (6) gets close to that of the reference voltage generator (4), and it is most stable when the two voltages get equivalent.
However, heretofore, according to the APC circuit indicated in FIG. 4 in the prior art, though it is capable of regulating the output intensity of laser diode (6), it cannot manipulate the output intensity. To adjust the different intensity, it requires an apparatus such as an optical attenuator.
In the higher accuracy measurement such as of the loss characteristics of optical fiber or the level characteristics of optical devices, the results tend to be affected by the intensity level of light source to be employed for it. Heretofore, to introduce variations in input intensity level in the prior art, it has been a practice to insert the various optical attenuators in the measurement system or to change resistance of negative feedback of the current-voltage converter (2) in FIG. 4. However, there have been a problem in case of employing the optical attenuators, that since the connecting loss of light connector can vary from one measurement to another, a high enough accuracy could not be attained.
The prior art technique of changing the resistance of negative feedback circuit is disclosed, for example, in Japanese patent application Kokai No. 180346/85. However, in this case, it took some time and trouble to change the resistance for each measurement.